The present invention relates to toners for use in printers employing xerography techniques and, more particularly, to an improved combination toner for use in a xerography printer that permits improved transfer to developed images.
The term "xerography" as used herein refers to a dry photographic or photocopying process in which a negative image is formed on an electrically charged plate by a resinous powder and is electrically transferred to and thermally fixed as a positive image on a paper or other copying surface. Various types of printers utilizing xerography techniques have been developed.
Three types of toners are generally used for xerography printing. In printers utilizing "Carlson's process" insulating non-magnetic toners are used for Two-Component Magnetic Brush Developing as well as for Floating Electrode Effect Developing (FEED). Alternatively, insulating magnetic toners are used in the Jamping Developing Method and conductive magnetic toners are used in electrofacsimile machines.
Xerography techniques have been improved to a point that exposure and developement can be performed simultaneously to create images. This method is referred to as Direct Developing Process (DDP) and is a process that promises to greatly simplify image development. An example of such a technique is disclosed in Japanese Patent Laid Open Application No. 58-153957.
The best DDP image forming method requires the surface of an image forming member having a photoelectric conductive layer to be swept with a brush of conductive magnetic toner to which a bias voltage has been applied. The electric charge carried by the toner is different in the unexposed portion of the image forming member where the photoelectric conductive layer acts as an insulator than in the exposed portion where the photoelectric conductive layer acts as a conductor. The difference in the electric charges corresponds to a difference in electrostatic attractivity of the toner to the surface of the image forming member. Accordingly, a toner image is formed.
A significant shortcoming of the described toner is due to its conductivity which causes the toner charges to be neutralized in a short period of time and residual charges to be lost. Accordingly, the electrostatic attractivity of the toner to plain paper is decreased and it is difficult to completely print an image by known electrostatic printing methods.
Accordingly, it is desirable to provide a toner for use in a printer employing a direct developing process that overcomes the disadvantages of prior art toners.